Synchronous control of oscillators



Jan. 5, 1937. A. EI HARPER SYNCHRONOUS CONTROL OF OSCTIILLI'K'IORS FiledDec. 29, 1954 w T E B M R m FIG. 2

//v l EN TOR A. E. HARPER A TTORNEV Patented Jan. 5, 1937 SYNCHRONOUSCONTROL OF OSCILLATORS Augustus E. Harper, Rockville Centre, N. Y.,assignor to Bell Telephone Laboratories, Incorporated, New York, N. 2.,a corporation of New York Application December 29, 1934, Serial No.759,749

4 Claims. (Cl. 250-36) This invention relates to the synchronous controlof oscillators and to an oscillator adaptable for use in thesynchronizing system of the invention as well as having utility forgeneral applications.

The invention has expression in quite widely variant forms. Treated asan oscillator, per se, the invention inheres in the use of a so-calledvariable-mu tube to replace the more conventional type of. vacuum tubein vacuum tube oscillator circuits. The variable-mu principle, asimplying a more linear characteristic of the tube, and recognizing thatthe conditions for the initiation of oscillations and the character andamplitude of the oscillation products depend on this characteristic,means generally that a variable-mu tube oscillator has certainoutstanding advantages over oscillators using other types of tubes asmaking possible a more accurate predetermination of the above criteria.It is an object of the invention to achieve this result in theelectrical and mechanical design of vacuum tube oscillators.

According to another aspect of the invention and which is generic to thevariable-mu tube and all other known types of vacuum tubes employing ananode, cathode and a control element, the tube is conceived aspresenting a capacitance to a potential impressed between the controlelement and cathode, which is valued as a function of the potentialimpressed between the control electrode and the cathode. In this mannera tube may be treated as a capacitor whose capaci tance may be variedelectrically, as distinguished from mechanical capacitance varying meansof the prior art. It is therefore an object of the invention to achievea variation of capacitance wholly by electrical means with the attendantpositiveness, simplicity, and reproducibility inherent in this sort offunction.

A particular application of the above electrically variable capacitancefeature of the invention is in a synchronizing system in which thevariable potential to be reflected in a change in capacitance is causedto be a measure of an incipient difference in frequencies of thecontrolling and controlled waves and in which the variable capacitanceis an element of the frequency determining circuit of the oscillatorwhich produces the controlled wave. In the carrying out of thissynchronizing principle the two waves concerned are combined in anydesired or convenient type of. modulating device. One of the modulationproducts when the two component frequencies are the same but differ onlyin phase as initiating a relative change in frequency, is a. directpotential the value of which is a direct function of the difference ofphase. This potential is impressed between the control element andcathode of the controlled oscillator to achieve a compensatory variationof the frequency of the wave generated thereby so as to effectively locktogether the frequencies of the two waves. If the controlled oscillatorhas piezoelectric crystal stabilization the imputed variable capacitancemay be thought of as replacing or adding to the capacitance of theconventional trimming condenser placed in simple relation to thepiezoelectric crystal to effect a small change in the frequency of the.generated waves. If the controlled oscillator does not havepiezoelectric crystal stabilization but is, rather, a vacuum tubeoscillator whose frequency is principally determined by the electricalconstants of the circuit,- a similar change of frequency may be achievedsince the input capacitance .of the tube, in any except a veryapproximate analysis of the circuit functions, may be treated as acomponent part of the reactance of the frequency determining network,its relative effect depending on the relative values of the constants ofassociated circuit elements.

According to the aspect of the invention as presented in the next aboveparagraph it is an object of the invention to utilize thecharacteristics of a vacuum tube whereby its input capacitance is afunction of the impressed potential to synchronize the frequencies ofwaves from two wave sources one of which utilizes said tube andtherefore to constrain one wave source to generate oscillations havingthe frequency of the wave from another source without the interpositionof mechanical means to achieve the frequency change in the controlledsource.

In operation, the frequency of the controlled source (hereinafter to bedenominated oscillator) is adjusted manually while coupled with thecontrolling wave circuit until a condition of synchronism is evident bythe use of a critically positioned milliammeter as in accordance withconventional synchronizing practice, whereafter the two frequenciesshould keep in step. In the event that the frequency of the wave fromthe controlled oscillator should tend to drift away from thissynchronous relation, its phase relative to the phase of the controllingwave will correspondingly change, thereby producing or increasing thecontrol potential, which will then bring the controlled oscillator backinto synchronism. This arrangement, therefore, not only 1 holds thefrequencies in synchronism but also ho ds the phase of the controlledoscillator at a predetermined value relative to the phase of thecontrolling wave. While the choice of tube for this synchronizingfunction is not limited, the characteristics of a variable-mu tube whichserve to distinguish this type of tube generally from its prototypes,commend it especially for use in this connection.

Other objects of the invention will appear more fully hereafter inconnection with the following detailed description of the invention asillustrated by the accompanying drawing in which:

Fig. 1 illustrates a synchronizing system of the invention, includingthe variable-mu tube oscillator also of the invention;

Fig. 2 illustrates a form of the push-pull modulator alternative tothat-enclosed in dashed line boundaries in Fig. 1; and

Fig. 3 is a partially symbolic representation of the controlledoscillator of Fig. 1.

In the synchronizing system of Fig. 1, which is characterized by a novelmeans for coercing the frequency of the wave from the controlledoscillator to cause it to conform with that of the controlling wave, thepurpose generally is to conform the frequency of the wave fromcontrolled oscillator I to the frequency of the controlling wave whichis incident on the system, and indirectly on the controlled oscillatoritself, through circuit 4. The frequency of the wave here incident, andcorrespondingly the current or wave itself, will sometimes be describedby the word controlling and sometimes by the word standard. Thecontrolled oscillator, or more accurately the output circuit 3 in whichthe eventual controlled frequency energy appears after amplification ifdesired by amplifier 2 might well be used with a local broadcast stationconstrained to use a frequency having a very definite relationship,perhaps representing equality of frequencies, with a distant sourceassociated with the incoming circuit 4, although there is intended to beno limitation as to the use to which the controlled frequency is put sofar as concerns this invention.

The controlled oscillator I has no limitation as to circuit or structureexcept only that it employs an electric discharge tube 5 in theoperation of which its mutual conductance is a function of this gridpotential. This tube is illustrated as a. so-called variable-mu" tube,constituting an example of a tube having this function, althoughconventional tubes having the usual anode, cathode and controllingelectrode, would do substantially as well. The theory on which thisfunction is based, which function is a very important feature in theoperation of the synchronizing system of the invention as a whole, willbe analyzed later, in connection with the symbolic showing of thecontrolled oscillator circuit in Fig. 3. As used herein this tubecharacteristic supplies a variable capacitance requisite to determiningan adequate frequency change or control in conformity with changes inrelative frequency, or tendency thereto, of the controlled andcontrolling waves.

In the specific controlled oscillator illustrated a piezoelectriccrystal 6 is used to stabilize the frequency. As is customary in the artof piezoelectric crystal oscillators, the crystal is connectedeffectively between the controlling electrode and cathode andcooperates, by its equivalent reactance, with the equivalent reactanceof the output tuned circuit 1 and the inter-electrode capacitances toconstitute the circuit as a whole an equivalent Hartley type oscillator.The condenser 8 may be used, as is illustrated, in close relation to thecrystal, although it does not have to be in shunt to it, to makepossilie a slight change in the generated frequency although, of course,the crystal is the primary frequency determining element. A condenserused in this way is sometimes denominated a trimming condenser. Theeflect of the variable mutual conductance of the tube, as a function ofthe variable grid (controlling electrode) potential, is reflected in acorrespondingly variable tube input capacitance, as will be shown in thesubsequent analysis. This capacitance accordingly is in such a positionas to serve as the capacitance of a "trimming condenser to replace thecondenser 8 or as a secondary trimming condenser to provide thecomponent of frequency change required for synchronizing or otherpurposes, this not being inconsistent with the use of a primary trimmingcondenser to initially adjust the frequency within the range ofsynchronizing control. The function of the tube just described is notinconsistent with, or exclusive of its conventional function in anoscillator circuit, of regeneratively repeating and amplifying the waveimpressed on its input. In fact, the oscillator circuit as a whole isconventional as to its continuity although it is believed that applicantis the first one to have used a variable-mu tube, instead of alternativetypes of tubes, in such a circuit. A variable-mu tube and itscharacteristics are described in a paper by Ballantine and Snow in theDecember, 1930 number of the Proceedings of the Institute of RadioEngineers, as well as in British Patent 376,737, complete accepted July6, 1932.

The amplifier 2 is not an essential element of the system of theinvention although useful here as in other systems where it isconventionally used after a prime source of oscillations and especiallyafter a crystal stabilized oscillator. The amplifier here illustrated isentirely conventional and is merely typical of a considerable choice ofamplifiers, including also the coupling to the oscillator, that could bemade. It is not believed that a more detailed description is merited.

The variable direct potential by which the variation of tube capacitanceis achieved is impressed on the controlling electrode of the controlledoscillator tube by circuit 9. This potential is made to reflect anincipient relative change of the two significant frequencies by beingmade an output product of the combination or intermodulation of thecontrolling wave and the controlled wave in balanced modulator I0. Thismodulator comprises two tubes, here of the two-element type, inpush-pull relation. The controlling wave is impressed in the same phaseon the cold electrodes in the two tubes through circuit 4 and thecontrolled wave is impressed differentially on the same electrodesthrough circuit H connected to the output circuit 3 of the amplier 2.The use of two tubes here has the advantage of the use of two tubesgenerally in modulating circuits, as pertaining to the elimination bybalancing of certain undesired components and the accentuation of othercomponents. However, the principle of operation of the modulator, orcombining device, for the present considered purpose does notnecessitate the use of two tubes in push-pull relation as shown, howeverpractically desirable this alternative may be. Neither does it requirethe use of the two-element tubes disclosed as distinguished fromthree-element tubes. Nor do the two impressing wave circuits have to berelated to the tubes and to the output circuit 9 exactly as shown. Infact, in the alternative circuit of Fig. 2, the relations of the twoimpressing wave circuits have been interchanged as compared with therelation shown in Fig. 1. Likewise, this circult of Fig. 2 illustrateshow the principle may be adapted to the use of three-element tubes. Theprior art contains many disclosures of speciflc modulators, especiallybalanced modulators, which could be used alternatively. For example,Aifel 1,450,966, April 10, 1923, discloses, by the modulator M, apush-pull or balanced modulator utilizing three-element tubes whereinthe output wave is taken from the circuits including the anode elementsof the tubes. The Affel circuit generally is suggestive of that of applicants to the extent that it likewise develops a synchronizing variabledirect potential component from the modulating operation and uses it toachieve the same ultimate result although it does not coerce thecontrolled oscillator by electrically varying a capacitance as in theoperation of applicant's circuit.

The milliammeter or the like I! is so positioned as to be made use of indetermining when the two frequencies are approximately the same asmeasured by the character of the modulator output. For example, thecontrolled frequency may be adjusted manually by trimming condenser 8until approximate synchronization is indicated by the condition ofsteady reading of this milliammeter l2 whereafter the synchronizingprinciple should be effective to further maintain the equalityrelationship of the frequencies. When the frequencies are the same, thebeat frequency product. of the modulation which would otherwise be ameasure of the difference between the two frequencies, would take the.

form of a direct potential. As the two frequencies tend to varyrelatively this potential correspondingly cyclically tends to varythrough a cycle measuring a unit difierence of relative frequency.Accordingly, at this epoch, namely, at the initiation of the relativechange of frequency, the only reflection is a relative change of phasewhich is measured by a correspondingly variable direct potential, thevalue of which may be ob-' tained from a reading of the meter l2. whenthe controlling'wave has taken control of the frequency of the localsource it maintains it by reason of the coercive effect of this directpotential, which effect varies proportionally as the tendency toward arelative change of frequency and therefore obeys the basic law to whicha stable frequency system, including a synchronizing system of thiskind, must be subjected.

It is evident that the synchronizing system of the invention is highlysensitive to the extent that itemployselectric frequency varying meansas compared with the mechanical means of Fig. l of the Affel patent andof other alternative systems, in the prior art and that it, like theAifel system, is inherently quickly responsive to a relative change offrequency on account of its responsiveness, immediately, to the changeof phase which initiates such change of frequency, the maximum coerciveforce coming into play before the frequencies have differed by as muchas a cycle. As electrically varying a frequency by variation of acapacitance, instead of by a variation of inductance as in Fig. 2 ofAffel, or by other types of variation illustrated by the prior art,other practical advantages are obtained in the simple, positive, controlof the frequency of the wave from vacuum tube oscillator. Although thistype of control is particularly adaptable in the instance of .apiezoelectric crystal stabilized oscillator as illustrated, theprinciple is equally applicable to vacuumtube oscillators notcharacterized by the use of stabilizing crystals or the like, providedthat the generated frequency is a function of the reactance of afrequency determining circuit element, for in all such cases the inputtube capacity may be made effectively a part of a frequency determiningelement and therefore will exert a frequency determining effect.

Fig. 3 illustrates, partially symbolically, a controlled oscillator likethat of Fig. l although somewhat generalized. Therein the crystal 8 ofFig. 1 has been replaced by its analogous inductance l3 and, toillustrate how the principle may be effective in a circuit more closelyconforming with the conventional Hartley oscillator circuit than acrystal stabilized Hartley oscillator circuit, the outputcircuitlnductance H which may be taken as symbolizing theeflectiveinductance of the circuit 1 of Fig. 1 is illustrated as directly coupledwith the input inductance l3 alternatively to the coupling through theinter-electrode capacitance of the tube as assumed in the case of theoscillator of Fig." l and as would ordinarily be required in crystalstabilized oscillator circuits. The circuit otherwise is substantiallythe same as that of the oscillator of. Fig. 1. Since it is the purposeof this Fig. 3 to analytically demonstrate the effectiveness of anelectric discharge device as a variable capacitance, and since thisanalysis will require a consideration of'certain geometrical tubereactances, that is, inter-electrode reactances, the showing ispurposely made a little unconventional in order to indicate all of theelements to be made use of in the analysis. The elements indicated bydash lines represent these geometrical reactanbes of the tube. For thepurposes of the analysis the constants of the equations will be listedbelow with their definitions,. t hese quantities as occurring in theequations being also used in Fig. 3 to represent the structures havingthese characteristics;

Cz=a tuning condenser for determining the R As in accordance withwell-known tube theory:

R o a1+ av This equation reduces to:

'I'herefore the effective capacitance of the tube, namely, Cg, whichcapacity is introduced in parallel with the resonant frequencydetermining circuit constituted by inductance I 3 and condenser Ct isdetermined by the geometrical gridfilament and grid-plate capacitancesof the tube plus a factor which is a function of the amplification ofthe tube and included circuits. This demonstrates that a vacuum tube andcircuit, the amplification of which may be varied, may be used as acorrespondingly variable capacitance.

Since the resultant capacity C3 is a function of the geometrical tubecapacities Cgp and Cgf, it is apparent that if these capacities arepurposely minimized by special tube design as in shield grid tubes etc.,the available variable Cg may not be sufficient for control purposes. Insuch cases it will be necessary to artificially increase the capacityCgp by means of a small external condenser, connected between the gridand the plate. To further simplify, let it be assumed that R is small ascompared with R as it could easily be in practice. The variableamplification factor [LR R+R becomes approximately E 0 and since themutual conductance of the tube, is a function of the grid voltage in theconventional types of vacuum tubes having a plate, grid and filament,and also having additionally if desired shielding and space chargeelectrodes, and especially in a so-called variable-mu tube, theeffective input capacitance of the tube is a function of the gridpotential. The initial mutual conductance, and hence the inputcapacitance, may be adjusted by a corresponding adjustment of the gridbiasing resistance Rb which adjustment might be treated as supplementalto the corresponding adjustment of the condenser Ct to slightly vary thefrequency. The variation of frequency in the carrying out of thesynchronizing control function is accomplished by the subsequentvariations of grid potential as a function of the variation of relativefrequency as has been explained.

It will be obvious that the general principles herein disclosed may beembodied in many other organizations widely different from thoseillustrated without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

1. An oscillation generating system comprising in combination, anelectric discharge device having an anode, a cathode and a controllingelectrode and which is characterized by having a mutual conductance,hence a capacitance between cathode and controlling electrode, which isa function of the potential impressed between said cathode andcontrolling electrode, feedback means connected to said device forproducing oscillations, a resonant frequency determining circuitcomprising the effective condenser constituted by the cathode andcontrolling electrode of said device, the capacitance of which varies,as recited, with the variations of potential impressed therebetween, andmeans impressing a variable potential between said cathode andcontrolling electrode to correspondingly vary said capacitance and hencethe frequency of the generated oscillations.

2. A synchronizing system comprising the circuit recited in claim 1together with a controlling wave source, and means, responsive to avariation of relative frequency of the waves from said source andgenerator for establishing the variable potential impressed between thecontrolling elec trade and cathode of said device whereby the frequencyof the generated waves is constrained to at all times equal thefrequency of the controlling wave.

3. In a synchronizing system in combination, a controlled oscillatorcomprising an electric discharge device having an anode, a cathode, acontrol electrode and a resonant frequency determining circuit, in theoperation of which the mutual conductance and hence the inputcapacitance is a function of the difference of potential between thecontrol electrode and cathode, said input capacitance effectivelyconstituting an element of said frequency determining circuit, acontrollingwave source, means for intermodulating the waves from saidoscillator and said source, means for deriving from said intermodulatingmeans the difference frequency modulation prod uct, which becomes adirect potential varying with the difference in phase of the two wavewhen the difference in frequency is less than one cycle, and means forimpressing said difference frequency product between said controlelectrode and cathode, whereby a tendency toward a relative change ofthe two frequencies from equality ive rise to a corresponding forcetending to compensatorily vary the frequency of the controlledoscillator so as to perpetuate the initial condition of frequencyequality.

4. The synchronizing system specified in claim 3 in which the electricdischarge device is a variable-mu tube.

AUGUSTUS E. HARPER.

